Solar water heaters generally comprise a solar radiation collector, a water storage tank and a heat transfer system for transferring heat collected at the collector to the water contained in the storage tank. Water within the storage tank is progressively heated by heat transferred to it from the solar radiation collector. Typically, there is a supplementary heating device, such as an electrical heater, for heating the water in the storage tank in circumstances where there is insufficient solar radiation for such purposes, such as for example on cloudy days.
At times when there is insufficient solar radiation and supplementary heating is required, a considerable amount of energy can be expended in heating the volume of water contained in the storage tank, both in terms of available energy from solar radiation and energy delivered by the supplementary heating device. It can therefore be wasteful to heat the entire volume of water, particularly as there may only be a small volume of hot water required. Additionally, the time taken to heat the entire volume of water to a desired temperature can be considerable, so causing delays in the availability of even a small volume of hot water.
It is against this background, and the problems and difficulties associated therewith, that the present invention has been developed.